The present invention relates to an electrically heated melting furnace for the melting of mineral materials, such as frits, (vitreous) enamels and the like, wherein a bath of molten material contained in a tank is heated by means of electrodes contacting the molten material and having a current flowing therethrough which heats the molten material by Joule's heat, wherein a batch is fed onto the melt or molten bath, and the molten product is withdrawn from the bottom of the tank.
Furnaces are known for the melting of mineral materials and especially frits, (vitreous) enamels and the like, wherein the conventional, shallow bath of molten material is heated from above by the heat produced by gas or oil burners, and wherein the molten mass is withdrawn either directly through the bottom or through a lateral outlet including a riser and feeder. However, these conventional furnaces suffer from the drawback of releasing great quantities of emission to the environment, which emission includes particularly the exhaust gases of the burners and the, in part toxic, gases released from the batch. Further, it is advantageous in these furnaces that their volume is required to be large as compared to the capacity thereof, such that these furnaces are expensive to construct and such that their efficiency is low; this means that a great energy demand per unit of weight of molten material is unavoidable.
Still further, a furnace developed by the applicant is known, which furnace in addition to the conventional mode of heating by oil or gas burners, is also provided with a heating system in the form of electrodes immersed into the molten material; in an further development, such furnace may be operated with high efficiency. However, it is still of disadvantage in this prior furnace that a relatively shallow tank containing molten material is present in which the surface losses are not kept at the desired minimum level. The construction of this furnace is still relatively expensive. On the other hand, it is advantageous in this prior furnace as compared to the "conventional" furnaces that, owing to the electric heating, a smaller quantity of exhaust gas is released by the burners, and that all of the exhaust gas produced during the melting of the batch may be collected totally and, optionally upon cleaning and detoxication, released to the atmosphere. However, this necessitates an extremely high degree of expenditure.
In view of this, it is the object of the present invention to provide a melting furnace particularly for frits, enamels and other mineral materials, which does no longer suffer from the abovementioned drawbacks and which offers optimum efficiency, substantially the theoretical efficiency, with respect to consumption of thermal energy.
Furthermore, the furnace according to the present invention should be inexpensive, of compact and small-size construction, while nevertheless offering an extremely high capacity, namely an extremely high rate of throughput. Operation of this furnace should be controllable without difficulty and in easy manner, and the quality of the molten materials should be improved as compared to the quality obtained by conventional furnaces. The advantages of the complete withdrawal and removal of the exhaust gases should be retained, and, in particular, the flow through the outlets should lend itself to be controlled by most simple means independently of the level of the surface of the molten glass bath. Still further, the furnace according to the invention including the components thereof should have a long operational life, and reconstruction of the furnace should be possible readily and in a short period of time.
According to the present invention, this object is solved in that the furnace space or tank having the molten bath has a square or almost square cross-section or the cross-section of a regular polygon; that the depth of the furnace space or tank having the molten material therein corresponds almost to the diameter of the tank; and that a plurality of radiator heating rods are arranged above the batch material and said heating electrodes are positioned in the lower portion of the molten bath above the tank bottom.
In order to facilitate the outflow, the furnace is of such construction that said bath includes a lateral outlet to which a riser followed by a feeder are connected, with the bottom of said riser and/or of said feeder each being provided with a bottom outlet.
In particular, for the control of the required temperature of the outflowing material and expecially for the control of the quantity discharged per unit of time, the bottom outlet is formed in such a manner that it includes a cylindrical portion or element and a further underlying, concetrically disposed, circular disc-shaped or annular portion or element of a refractory material being conductive at high temperature and acting as an electrical conductor, which elements are conductively coupled to a current supply line.
In order that the required temperature of the refractory materials can be controlled, that these materials can be supported or retained and in order to provide for tightness of the outlet and, further, to control the fritting of the outflowing materials within the outlet, this bottom outlet may be characterized by the fact that the current disc-shaped element is retained in an metallic annular element and supported by the bottom plate thereof; that an annular cooling passage is provided below said bottom plate; that said bottom plate has a center aperture including a bell-shaped extension in the region of said annular cooling passage; and that radial channels or passages for the coolant extend from said annular cooling passage into the bell-shaped extended portion of said center aperture.
In order to provide for versatile cooling and thereby to allow for adequate cooling also of the edge portions of the outlet and of the adjacently disposed refractory materials which need not be (electrically) conductive, beneath the bottom plate and adjacent the edge thereof there is provided another annular cooling water passage, and for the insulation against the cooler ambient air as well as for obtaining a particularly compact structure, both the circular disc, the mounting means thereof, said annular cooling passage and said annular cooling water passage are provided within a aperture in the refractory material of the tank bottom.
In order to obtain a defined outflow, and in order to improve the electrical (current) paths within the outflowing material, preferably the center aperture of said cylindrical portion is of substantially greater diameter than the center aperture of said circular disc-shaped element or portion, while, in order to avoid excessive heating of the adjacent material and to confine the current paths to the associated region within the refractory material, it is preferable that the cylindrical portion is inserted into a hole or aperture defined by refractory material being non-conductive or of substantially lesser conductivity than the material of said cylindrical portion.
In view of the sum of the advantages that may be achieved by the invention, particularly of the control being substantially improved as compared to the conventional furnaces, of the improved efficiency and of the improved quality of material, it may be spoken of an ideal solution to the existing problems by the subject matter of the invention.